1. Field of the Invention
The present invention relates to the field of laser processing methods and systems, and specifically, to laser processing methods and systems for laser processing multi-material devices.
2. Description of the Related Art
Lasers can be used in the processing of microstructures in memory and integrated circuit devices. For example, laser pulses can be used to ablate conductive links or link portions in a memory device, such as DRAMs in order to substitute working redundant memory cells for defective memory cells during memory manufacture.
Recently, the use of new materials, such as aluminum, gold, and copper, coupled with the small geometry of these devices, have made the problem of link removal more difficult. Economics and device performance goals have driven the size for the DRAMs and logic devices to very small physical dimensions. Thus, it can be increasingly difficult to irradiate a target structure without damaging surrounding components such as the substrate and adjacent circuitry and links. Furthermore, as more links need to be processed for a given area of semiconductor circuitry, the time required to process a given die increases.
When a single laser pulse or burst of pulses is used to irradiate and sever each link designated for removal, the beam path of laser pulses may move relative to the substrate during the process of irradiation in an “on-the-fly” link blowing process. This relative movement may include moving the substrate and/or moving the beam, although substrate motion on an X-Y stage in conjunction with a vertically oriented and stationary beam is a currently common approach. In conventional laser processing systems, groups of arrayed microstructures are processed. The array may be links in a row, links in closely spaced rows, links in staggered rows and similar regularly spaced arrangements. The conventional processing is generally carried out with either an energy on demand system (e.g. pulse equalization) or an energy picking system (e.g. pulse picking). In the energy on demand system, an irradiation period is timed to coincide with a moving target and the processing rate is limited by a minimum period between energy on demand irradiation periods. In the energy picking system, the laser is pulsed in a continuously repeating sequence at a predetermined repetition rate (e.g. at a q-rate, pulse rate, or burst rate) and the arrayed microstructures in a group are moved synchronously with the repetition rate so that energy is available to process any microstructure in a particular group. The processing rate is limited by a period associated with the maximum repetition rate, and an acousto-optic device or other optical switching device blocks energy from reaching the substrate except when processing a selected synchronized target.
The conventional energy picking process is illustrated in FIGS. 1 and 2. A repeating sequence of laser pulses, for example pulses from a q-switched laser, pulses from a sequence of pulse bursts, or a sequence of temporally shaped pulses is generated at a predetermined repetition rate. A group of links 10 having a characteristic spacing d is put in motion relative to a processing head at a predetermined velocity V by moving a stage 12 under control of a control computer 14. As adjacent links move relative to the processing head, there is an associated transit time T1 such that after a period equal to T1, the substrate has moved by an amount equal to the characteristic spacing of the links. Put another way, the link to link period at velocity V relative to the processing head is T1.
In a conventional processing system links and pulses are synchronized. T1 and the period of the laser pulse repetition rate (e.g. the pulse to pulse period of a q-switched laser controlled by trigger signals from the control computer 14) are made equal. With this method, a pulse is available to process every link. Pulses that are synchronized with links to be processed, such as links 10a, 10d, and 10f FIG. 2, are allowed to reach the targets and process the respective links. Pulses that are synchronized with links that are to remain intact are blocked from reaching the targets by an energy control and pulse picking system 16 of FIG. 1, as indicated by dashed circles in FIG. 2 where the beam would strike if it was not blocked.
It will be appreciated that the time required to process a given set of links within a group of a row or a column of links is approximately the number of links times the time period T1, which in these systems equals the laser pulse repetition rate. If the laser used has a maximum pulse rate of 50 kHz, for example, completing the pass of the beam across the 11 links of FIG. 1 will require at least 200 microseconds.
Although the above embodiment was described in terms of single pulse link processing, link blowing systems have been described that apply multiple pulses to each link to sever the link. FIG. 3 shows a system which applies a burst or sequence of two pulses to each link. In this embodiment, the pulse selector 16 selects groups of pulses rather than individual pulses for link processing. In some embodiments, the laser itself produces separated bursts of pulses where the pulse to pulse separation within the burst is much less than the separation between bursts. In these embodiments, the pulse picker 16 selectively passes or blocks pulse bursts. Other known embodiments use multiple lasers or split and re-combined pulses to produce a variety of intensity profiles of the laser energy applied to a link for processing. It will therefore be appreciated that all of the discussion throughout this document related to applying a pulse to a target structure for processing includes applying a sequence of pulses, pulse groups, combined pulses, or pulse bursts, or any other irradiance intensity profile for performing a complete or partial target processing function.
In many advantageous embodiments, the pulse picker 16 is an acousto-optic modulation device, but may be an electro-optic switch, a fast steering mirror or any other type of optical switch with sufficient speed and accuracy.
Other uses in addition to pulse picking for acousto-optic modulators, fast steering mirrors, or other forms of high speed deflectors have also been described. One such use is for beam position correction along the direction of beam motion if a long pulse or pulse burst is being applied to process a link. Without correction, if the pulse or pulse burst is short relative to the transit time of the beam spot across the link during the pulse or pulse burst duration, the beam spot will not move appreciably during the above described relative beam and link motion during the on-the-fly processing. However, as shown in FIG. 4, if the burst has many pulses or includes relatively long inter-pulse spacing, the beam spot can walk off the center of the link 18 over the course of the pulse sequence. In U.S. Patent Publication 2002/0167581 to Cordingley et al., a deflector (e.g., a high-speed deflector) is described to deflect laser pulses to improve the coincidence of the pulses with the target structures in this situation. As described in this Publication, the deflector can act to oppose the relative movement of the beam spot across the link. In one embodiment generally illustrated in FIG. 5, the deflector 20 would be operatively coupled to the relative positioning system. The deflector 20 is preferably solid state and may be a single axis acousto-optic device which has a very fast “retrace”/access time. Alternatively, a higher speed electro-optic deflector (e.g., a gradient index reflector or possibly a digital light deflector) may be used. The time-bandwidth product (number of spots) can be traded for response time on an application basis. Alternatively, an electro-optic modulator may be used with a separate acousto-optic deflector operated in a “chirp mode” (e.g., linear sweep as opposed to random access mode) and synchronized (triggered) based on the positioning system coordinates. A modulator 22 may be used for intensity control and pulse gating/selection, to select pulses 24 at times t1, t2, t3 for target structure processing. As the beam path moves further across and beyond an edge of a target structure during a sweep of a multiple pulse processing function, the deflector 20 would more strongly deflect the beam path in a direction opposing the relative movement. Thus, a plurality of pulses would irradiate approximately the same portion of the target structure. U.S. Publication Number 2002/0167581 is hereby incorporated by reference in its entirety.
Another application of an acousto-optic modulator in a beam path of a laser processing machine is also described in U.S. Publication Number 2002/0167581. In an embodiment described therein with reference to FIG. 20, an acousto-optic modulator is used as a beam splitter to allow the processing of more than one target structure at a time. The beam paths of the resulting beams may be controlled by controlling the frequencies applied to the acousto-optic modulator to simultaneously position each resulting beam accurately on the targets to be processed.
Although high speed beam scanning within a dominant fundamental beam trajectory has been utilized in a variety of contexts such as described above, additional uses of high speed deflectors in link blowing systems would be useful in the field.
For further reference, the following co-pending U.S. applications and issued patents are assigned to the assignee of the present invention, describe many additional aspects of laser link blowing, and are hereby incorporated by reference in their entirety:                1. U.S. Pat. No. 5,300,756, entitled “Method and System for Severing Integrated-Circuit Connection Paths by a Phase Plate Adjusted Laser beam”;        2. U.S. Pat. No. 6,144,118, entitled “High Speed Precision Positioning Apparatus”;        3. U.S. Pat. No. 6,181,728, entitled “Controlling Laser Polarization”;        4. U.S. Pat. No. 5,998,759, entitled “Laser Processing”;        5. U.S. Pat. No. 6,281,471, entitled “Energy Efficient, Laser-Based Method and System for Processing Target Material”;        6. U.S. Pat. No. 6,340,806, entitled “Energy-Efficient Method and System for Processing Target Material Using an Amplified, Wavelength-Shifted Pulse Train”;        7. U.S. Pat. No. 6,483,071, entitled “Method and System For Precisely Positioning A Waist of A Material-Processing Laser Beam To Process Microstructures Within A Laser-Processing Site”, filed 16 May 2000, and published as WO 0187534 A2, December, 2001;        8. U.S. Pat. No. 6,300,590, entitled “Laser Processing”; and        9. U.S. Pat. No. 6,339,604, entitled “Pulse Control in Laser Systems.”        10. U.S. Pat. No. 6,639,177, entitled “Method and System For Processing One or More Microstructures of A Multi-Material Device”        
The subject matter of the above referenced applications and patents is related to the present invention. References to the above patents and applications are cited by reference number in the following sections.